News of a $400m Franco-Libyan arms deal has reignited a controversy in France about how the release by Tripoli of jailed medics was secured last week.

The six were freed after a high-profile visit to Libya by President Nicolas Sarkozy's wife, Cecilia.

The day after their release, the president himself flew to Libya to sign trade and health deals.

Since then, opposition socialists have demanded to know whether the Bulgarian detainees were part of the package.

Mr Sarkozy strongly denies that there was any such quid-pro-quo.

But the arms agreement - the first between Libya and a Western country since the EU lifted a ban in 2004 - has fuelled demands for explanations.

The deal was first outlined on Thursday by Saif Gaddafi, the son of the Libyan leader.

He told Le Monde newspaper that the arms sales had long been at the heart of talks between Libya and France.

In a democracy like ours, transparency must be the rule

Francois HollandeFrench Socialist Party leader

Later Libyan officials provided details, saying Tripoli had agreed to buy Milan anti-tank missiles and radio communications equipment from subsidiaries of the European defence and aerospace group EADS.

To Socialist Party leader Francois Hollande, the way the news emerged is in itself suspicious.

"How can we tolerate in a democracy that an arms deal should be announced by Gaddafi's son?" he told AFP news agency on Friday.

Mr Hollande said he had made inquiries about the agreements with the foreign and defence ministries and was told they knew nothing of them.

He called for a parliamentary inquiry into negotiations between Libya and France.

The investigation should "shed light on whether this was a regular trade deal", Mr Hollande added.

"In a democracy like ours, transparency must be the rule."

Doubts

Mr Sarkozy's spokesman David Martinon again denied that the issue of the jailed medics - who had been convicted of infecting children with HIV - played a role in negotiations.

Defence Minister Herve Morin, for his part, said the deals had been under discussion for months, and that a special committee on arms sales had cleared them in February - three months before Mr Sarkozy's election.

But the left-wing opposition is clearly not satisfied with the official explanation.

"Libya is not a democratic country and at least we could have been informed in parliament of the negotiations going on," Socialist MP Marisol Touraine told the BBC.

The government can expect a rough ride when parliament reconvenes next month.

In the meantime, the government is facing intense pressure from sections of the media.

"Doubts remain," Le Monde newspaper writes in an editorial on Friday.

"By trying to convince us that the new president and his wife pulled off the happy outcome, on their own, by their sheer persistence in negotiations, the Elysee was inviting a backlash."